Arrayed distributed acoustic sensing using single-photon detectors

ABSTRACT

In some embodiments, a method and apparatus, as well as an article, may operate to determine downhole properties based on detected optical signals. An optical detection system can include a fiber optic cable having a sensing location to generate a backscattered Rayleigh signal representative of measurement parameters. The optical detection system can further include a light source to transmit a measurement signal to cause the sensing location to provide the backscattered Rayleigh signal. The optical detection system can further include an optical detector comprising a single-photon detector (SPD) for detecting the backscattered Rayleigh signal received over the fiber optic cable. The optical detection system can further include circuitry to produce an acoustic signal representative of a downhole property based on the phase of the backscattered Rayleigh signal. Additional apparatuses, systems, and methods are disclosed.

PRIORITY

The present application is a U.S. National Stage patent application ofInternational Patent Application No. PCT/US2016/049021, filed on Aug.26, 2016, the benefit of which is claimed and the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND

Understanding the structure and properties of geological formations canreduce the cost of drilling wells for oil and gas exploration.Measurements made in a borehole (i.e., downhole measurements) aretypically performed to attain this understanding, to identify thecomposition and distribution of material that surrounds the measurementdevice downhole. Optical detectors are often used to perform thesemeasurements. Optical detectors use fiber optic cables that have greatertemperature capability, corrosion resistance and electromagneticinsensitivity as compared to some other types of energy conductors, suchas wires or cables. However, optical detectors are still subject tovarious noise sources that can reduce accuracy and reliability ofmeasurements taken with optical detectors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an optical detection system including anoptical detector and a cooling system including a cold head for coolingthe optical detector in accordance with various embodiments.

FIG. 2 is a block diagram of a single integrated optical chip includinga group of optical detectors dedicated to distributed temperaturesensing (DTS) and another group dedicated to distributed acousticsensing (DAS) in accordance with various embodiments.

FIG. 3 is a block diagram of an optical detection system including anoptical detector and a cooling system, without an intervening cold head,for cooling the optical detector in accordance with various embodiments.

FIG. 4 is a block diagram of an optical detection system including aswitching mechanism to direct optical signals to an optical detector inaccordance with various embodiments.

FIG. 5 is a flow chart of an example method of optical sensing inaccordance with some embodiments.

FIG. 6 is a diagram of a wireline system embodiment.

FIG. 7 is a diagram of a drilling rig system embodiment.

FIG. 8 is a diagram of a permanent monitoring system embodiment.

FIG. 9 a block diagram of an optical detection system including asuperconducting nanowire single-photon detector (SNSPD) in accordancewith various embodiments.

FIG. 10A illustrates an SNSPD using meandering nanowires in accordancewith various embodiments.

FIG. 10B illustrates an enlarged view of a superconductive portion ofthe SNSPD of FIG. 10A.

FIG. 11 illustrates an SNSPD having parallel pixels in accordance withvarious embodiments.

FIG. 12 illustrates a multilayer SNSPD structure in accordance withvarious embodiments.

FIG. 13 is a flow chart of an example method of optical sensing with anSNSPD optical detector in accordance with various embodiments.

FIG. 14 is a block diagram of an arrayed DAS system in accordance withvarious embodiments.

FIG. 15A illustrates an SPD array for fiber imaging in accordance withvarious embodiments.

FIG. 15B illustrates a grid of SPD arrays in accordance with variousembodiments.

FIG. 16 is a block diagram of a balanced receiver for retrieving phaseinformation from a backscattered Rayleigh signal in accordance withvarious embodiments.

FIG. 17 is a flow chart of an example method of estimating a downholeparameter in accordance with various embodiments.

DETAILED DESCRIPTION

Noise sources interfere with downhole measurement systems and can causedeterioration in signal-to-noise ratios (SNRs) of measurement signals.Thus, ongoing efforts are directed to reducing noise to improvesignal-to-noise ratios in optical detection systems. For example, SNRcan be increased by modifying certain parameters (e.g., resolution,fiber depth, and repetition rate). However, modification of theseparameters can reduce the accuracy of the optical detection system. Toaddress these concerns and others, systems, apparatuses, and methodsdescribed herein provide for cooling optical detectors to ultra-lowtemperatures (e.g., below 210 degrees Kelvin, below 70 degrees Kelvinor, in some embodiments, below 4 degrees Kelvin). This cooling canreduce or effectively eliminate thermal noise, thereby raisingmeasurement signal SNRs without changing other parameters, in opticalsensors for optical sensing.

FIG. 1 is a block diagram of an optical detection system 100. An opticaldetection apparatus 101 of the optical detection system 100 includes anoptical detector 102 for detecting light received through a fiber opticcable 104. In some embodiments, the light includes wavelengths in avisible range of wavelengths. However, embodiments are not limitedthereto and the light can include wavelengths in an infrared range ofwavelengths and/or in an ultraviolet range of wavelengths.

In some embodiments, the optical detector 102 includes a low-lightdetector (LLD) or an extremely low-light detector (ELLD). In someembodiments, the optical detector 102 includes a single-photon detector(SPD). In some embodiments, the optical detector 102 includes anavalanche photodiode. In some embodiments, the optical detector 102includes carbon nanotubes or other nano structures. However, embodimentsare not limited to these example optical detectors and other types ofoptical detectors can be used. For example, the optical detector 102 caninclude an integrated optical chip such as a silicon photonic resonatoror a focal planar array detector, among other optical detector types.

In some embodiments, the integrated optical chip can support multipleSPDs with each detector (or group of detectors) dedicated to a specificsensing type. For example, as shown in FIG. 2, a single integrated chip200 can include a group of optical detectors 202 dedicated todistributed temperature sensing (DTS) and another group 204 dedicated todistributed acoustic sensing (DAS). The DTS group can be further splitinto sub-groups of optical detectors that can sense scattered light (asopposed to incident light) components, including one group 208 ofoptical detectors for sensing an anti-Stokes signal and another group206 of optical detectors for sensing the Stokes signal of scatteredlight. Likewise, the DAS group can be split into a set 212 of opticaldetectors that sense one interferometric output (the in-phase) and a set214 that detects the other output (e.g., the quadrature). Other systemscan be provided such as distributed strain sensing (DSS) systems. Insome embodiments, the optical detector 102 can include additionaloptical elements to improve signal to noise ratios. These additionaloptical elements can include lenses, filters, mirrors, mixers,wavelength demultiplexers, and other elements.

Referring again to FIG. 1, the optical detection apparatus 101 caninclude a housing 106 for enclosing the optical detector 102 and forproviding an optical shield for the optical detector 102. The housing106 can include an aperture 108 for passage of the fiber optic cable104. However, embodiments are not limited thereto, and in someembodiments, a coupler can be mounted so that the fiber optic cable 104terminates at a boundary of the housing 106. The optical detectionsystem 100 includes a light source 110, separate from the opticaldetector 102 and the housing 106, for providing (e.g., “transmitting”)light through the fiber optic cable 104. In some embodiments, the lightsource 110 can provide light through one fiber optic cable 104 and lightcan be received by the optical detector 102 on a separate fiber opticcable 104, although embodiments are not limited to any particular numberof fiber optic cables 104 or combinations thereof. In some embodiments,the light source 110 can include additional optical componentsconfigured to provide or delivery the appropriate light to the systemincluding filters, mixers, non-linear crystals, timing chips, wavelengthmultiplexers, etc. In some embodiments, the light source 110 can bedownhole and in some embodiments, the light source 110 can be on thesurface. In some embodiments, multiple light sources can be provided onthe surface, downhole, or a combination thereof.

The optical detection apparatus 101 includes a cooling mechanism 112having the housing 106 mounted thereto. The cooling mechanism 112 isconfigured to maintain the temperature of a light-sensitive region ofthe optical detector 102 within a temperature range below 210 degreesKelvin. In some embodiments, the cooling mechanism 112 operates usingliquid helium (He) or liquid nitrogen (N₂). In some embodiments, thecooling mechanism 112 maintains the temperature of the light-sensitiveregion of the optical detector 102 at a temperature at or below 80degrees Kelvin. In some embodiments, the cooling mechanism 112 maintainsthe temperature of the light-sensitive region of the optical detector102 at a temperature at or below 5 degrees Kelvin (e.g., when sealedhelium systems are used). In some embodiments, the cooling mechanism 112can be of one or more of a variety of configurations, includingDilution-Magnetic, Collins-Helium Liquefier, Joule-Thomson,Stirling-cycle cryocooler, self-regulated Joule-Thomson, Closed-CycleSplit-Type Stirling, Pulse Tube, a two-stage Gifford-McMahon cryogeniccooler or multi-stage Gifford-McMahon cryogenic cooler, or a coolerusing magnetocaloric effect, by way of example. Lowering the temperatureof the optical detector 102 improves the SNR of the optical detector 102by decreasing dark current, by increasing sensitivity, and by reducingresistive loss by causing the optical detector 102 to enter asuperconducting regime of operation. In some embodiments orconfigurations non-SPD optical detectors 102 will not enter asuperconducting regime, while still having little to no thermal noise.

In some embodiments, the optical detection apparatus 101 includes a coldhead 114 between the optical detector 102 and the cooling mechanism 112.However, some embodiments do not include a cold head 114. FIG. 3 is ablock diagram of an optical detection system 300 including an opticaldetector 302 and a cooling mechanism 312, without an intervening coldhead, for cooling the optical detector 302 in accordance with variousembodiments. As with the optical detection system 100, the opticaldetection system 300 includes a light source 310, and a fiber opticcable 304 coupled between the downhole unit 316 and the optical detector302. The optical detector 102 can be encased in a housing 306 with orwithout an aperture 308 for permitting passage of the fiber optic cable304.

Referring again to FIG. 1, in some embodiments, the housing 106 ismounted to the cooling mechanism 112 such that moisture is preventedfrom entering the housing. For example, the housing 106 can be mountedsuch that a vacuum seal is formed with the cooling mechanism 112 or thecold head 114. The housing 106 can have a non-reflective inner surface.

The optical detection system 100 can further include a downhole unit 116(e.g., a downhole tool or a downhole sensor) configured to provide anoptical signal over the fiber optic cable 104. The optical signal can bea pulsed signal originating from distributed sensing, or a continuoussignal, among other signals.

The optical detection system 100 can include more than one opticaldetector 102 (shown within the dashed box in FIG. 1). At least oneoptical detector 102 can detect light received through the fiber opticcable 104 from more than one downhole unit 116. The optical detectionsystem 100 can include optical detectors 102 of more than one type. Forexample, some optical detectors 102 in the optical detection system 100can be SPDs, avalanche photodiodes, integrated optical chips, etc. Someoptical detectors 102 can be non-LLD detectors, or non-ELLD detectors(e.g., other than non-SPD detectors). Non-LLD detectors can includep-i-n (or PIN) photodiodes, phototransistors, photovoltaics,photoconductors. Downhole units 116 can include units configured todetect temperature. Other downhole units 116 can be coupled to the sameoptical detector 102 or to a group of optical detectors 102, or in anyother configuration. Downhole units 116 can detect other properties andperform other functions such as acoustic sensing, resistivitymeasurements, etc. Some optical detectors 102 may not be mounted on anycooling mechanism 112, some optical detectors 102 may be mounted withother optical detectors 102 on a same cooling mechanism 112, or theoptical detection system 100 can include more than one cooling mechanism112 arranged in any configuration with one or more optical detectorsmounted thereto.

As shown in FIG. 4, an optical detection system 400 can further includea switching or splitting mechanism 402 to direct optical signals to anoptical detector 404, 406 based on wavelength of the optical signal,power of the optical signal, polarization, or any other parameter orcriterion. For example, high-power optical signals 403 can be routed tonon-SPD optical detectors 404, and away from SPDs 406 and low-poweredoptical signals 405 can be routed to SPD optical detectors 406. Thisrouting can be performed to prevent damage to SPD optical detectors 406while still taking full advantage of LLD and ELLD capabilities of SPDoptical detectors 406. As will be appreciated, high-power opticalsignals 403 can cause saturation in SPDs 406, leading to damage to theSPDs 406 or to inaccurate results. In some examples, saturation of SPDs406 can occur with optical signal inputs having a power of about 100microwatts, and damage can occur at about 10 milliwatts. The noise floorthat can be detected by the SPD 406 can be at a level slightly belowsaturation level but is typically at least 20-30 dB. The saturationlevel and noise floors for non-SPD optical detectors 404 may bedifferent from the saturation level and noise floors for SPDs 406. Thesaturation levels and noise floors also may or may not overlap, and thusmultiple types of detectors may be used that can cover the full powerrange required for system measurement. For at least these reasons, tomeasure a larger range of possible optical signals, SPD opticaldetectors 102 are used in a system with non-SPD optical detectors 102.Splitting mechanisms 402 can direct or reroute optical signals based onpower level or other criteria, to take advantage of the different powerranges measurable by SPD optical detectors 102 versus non-SPD opticaldetectors 102.

In addition to or instead of a splitting mechanism 402, the opticaldetection system 100 can include a coupling mechanism or other mechanismto split the light with optical couplers (with or without feedback).These mechanisms can be multi-stage (e.g., the light can be split in onestage, then split again in a second stage), and can split light based onpower, wavelength, or phase. Processor or computation-based systems canalso be used in some embodiments to dynamically direct or reroute lightsignals among any available optical path as power increases, or based onany other criteria.

Referring again to FIG. 1, the optical detection system 100 can furtherinclude a measuring device 118 (which may be part of a computerworkstation) coupled to the optical detector 102 to obtain measurementdata, with a display 120 to display a graphical representation of themeasurement data. In some embodiments, some portions of the opticaldetection system 100 can be positioned at a surface of the Earth, whilesome portions to the optical detection system 100 can be placeddownhole. When more than one optical detector 102 is used, for example,some of the optical detectors 102 can be placed downhole, and some canbe placed at the surface. In some embodiments, one or more coolingmechanisms 112 can be placed downhole proximate one or more opticaldetectors 102 although power and geometry considerations should be takeninto account with such configurations to provide power for cooling in anappropriately sized borehole. In some embodiments, the measuring device118 and display 120 will be situated at the surface of the Earth, forexample as shown in FIGS. 6 and 7 described later herein.

FIG. 5 is a flow chart of an example method 500 of optical sensing inaccordance with some embodiments. The example method 500 can beperformed by the optical detection system 100 (FIG. 1) or by componentsthereof. The example method 500 begins with operation 502 with couplingan optical sensing apparatus to a downhole unit 116 (e.g., a downholesensing device) through a fiber optic cable 104. The optical sensingapparatus can include at least one optical detector 102 within a housing106. However, embodiments are not limited to one optical detector 102.The optical detector 102 can be cooled to a temperature below 210degrees Kelvin by a cooling mechanism 112. In some embodiments, theoptical detector 102 can be cooled to a temperature below 5 degreesKelvin.

The example method 500 continues with operation 504 with the opticaldetector 102 receiving optical signals from the downhole sensing deviceover the fiber optic cable 104. In some embodiments, the downholesensing device includes an intrinsic fiber optic sensor. In otherembodiments, the downhole sensing device comprises at least one fiberBragg grating or some other reflector. In at least these embodiments,the example method 500 can further include providing an optical signalto the intrinsic fiber optic sensor and receiving a reflected orbackscattered optical signal, responsive to providing the opticalsignal, that represents at least one downhole property. In embodiments,the backscattered signal can include Stokes and anti-Stokes components,or Raleigh components.

In embodiments, many optical signals can be multiplexed onto the fiberoptic cable 104. In at least these embodiments, the example method 500can further include de-multiplexing the optical signals at a switchingmechanism, and providing the de-multiplexed signals on at least twoseparate paths to at least two separate optical detectors 102.

The example method 500 continues with operation 506 with the opticaldetection system 100 detecting at least one downhole property based onthe optical signals. For example, the optical signal can be used todetect different properties of downhole structures, to provide opticalanalysis of fluid and material composition in a borehole or annulus, toperform geosteering, to determine values for porosity or composition ofthe borehole wall, etc.

FIG. 6 is a diagram of a wireline system 600 embodiment. The wirelinesystem 600 can comprise portions of a wireline logging tool body 602 aspart of a wireline logging operation. Thus, FIG. 6 shows a well duringwireline logging operations. In this case, a drilling platform 604 isequipped with a derrick 606 that supports a hoist 608.

Drilling oil and gas wells is commonly carried out using a string ofdrill pipes connected together so as to form a drilling string that islowered through a rotary table 610 into a wellbore or borehole 612. Hereit is assumed that the drilling string has been temporarily removed fromthe borehole 612 to allow a wireline logging tool body 602, such as aprobe or sonde, to be lowered by wireline or logging cable 614 into theborehole 612. Typically, the wireline logging tool body 602 is loweredto the bottom of the region of interest and subsequently pulled upwardat a substantially constant speed.

During the upward trip, at a series of depths instruments (e.g.,downhole units 116 described above with reference to FIG. 1) included inthe wireline logging tool body 602 can be used to perform measurementson the subsurface geological formations adjacent the borehole 612 (andthe wireline logging tool body 602). The measurement data can becommunicated to a surface logging facility 616 for storage, processing,and analysis. The logging facility 616 can be provided with electronicequipment for various types of signal processing, which can beimplemented by any one or more of the components of the opticaldetection system 100 (FIG. 1). Similar formation evaluation data can begathered and analyzed during drilling operations (e.g., during LWDoperations, and by extension, sampling while drilling).

The wireline logging tool body 602 is suspended in the wellbore by awireline cable 614 that connects the tool to a surface control unit(e.g., comprising a measuring device 118 (which can include aworkstation and have a corresponding display 120). This wireline cable614 can include (or perform functionalities of) the fiber optic cable104 (FIG. 1). The tool can be deployed in the borehole 612 on coiledtubing, jointed drill pipe, hard wired drill pipe, or any other suitabledeployment technique.

In addition to wireline embodiments, example embodiments can also beimplemented in drilling rig systems. FIG. 7 illustrates a drilling rigsystem 700 embodiment. The system 700 can include a downhole unit 116 aspart of a downhole drilling operation.

Referring to FIG. 7, it can be seen how a system 700 can also form aportion of a drilling rig 702 located at the surface 704 of a well 706.The drilling rig 702 can provide support for a drill string 708. Thedrill string 708 can operate to penetrate the rotary table 610 fordrilling the borehole 612 through the subsurface formations 714. Thedrill string 708 can include a Kelly 716, drill pipe 718, and a bottomhole assembly 720, perhaps located at the lower portion of the drillpipe 718.

The bottom hole assembly 720 can include drill collars 722, a downholetool 724, and a drill bit 726. The drill bit 726 can operate to createthe borehole 612 by penetrating the surface 704 and the subsurfaceformations 615. The downhole tool 724 can comprise any of a number ofdifferent types of tools including MWD tools, LWD tools, and others. Thedownhole tool can communicate with the logging facility 616. In someexamples, fiber optic cable 104 will be spliced, rerouted, coupled,guided, or otherwise modified to maintain connections at each drillcollar 722 and at each position along the drill string 708. In someembodiments, a fiber optic connector can be provided at each drillcollar 722 or other joint or position downhole. In some embodiments, thefiber optic cable 104 can be placed inside a steel casing 725 as shownin FIG. 7, outside the casing 725 (as shown in FIG. 8 and describedbelow), inside or outside of a production tube, inside or outside ofcoiled tubing, on a wireline cable, or in any other placement,configuration, or combination thereof. In some embodiments, downholeunits 116 can be placed in a repeater configuration or in an amplifierconfiguration to improve signal strength at the surface. In yet otherembodiments, fiber can be deployed continuously through a drill stringusing a dart, a torpedo, a reel, a feedthrough, or some other deploymentdevice.

During drilling operations, the drill string 708 (perhaps including theKelly 716, the drill pipe 718, and the bottom hole assembly 720) can berotated by the rotary table 610. Although not shown, in addition to, oralternatively, the bottom hole assembly 720 can also be rotated by amotor (e.g., a mud motor) that is located downhole. The drill collars722 can be used to add weight to the drill bit 726. The drill collars722 can also operate to stiffen the bottom hole assembly 720, allowingthe bottom hole assembly 720 to transfer the added weight to the drillbit 726, and in turn, to assist the drill bit 726 in penetrating thesurface 704 and subsurface formations 714.

Thus, it can be seen that in some embodiments, the systems 600, 700 caninclude a drill collar 722, a downhole tool 724, and/or a wirelinelogging tool body 602 to house one or more downhole units, similar to oridentical to the downhole units 116 providing information over the fiberoptic cable 104 and illustrated in FIGS. 1 and 2.

Thus, for the purposes of this document, the term “housing” when used toaddress tools below the surface (e.g., downhole), can include any one ormore of a drill collar 722, a downhole tool 724, or a wireline loggingtool body 602 (all having an outer wall, to enclose or attach tomagnetometers, sensors, fluid sampling devices, pressure measurementdevices, transmitters, receivers, acquisition and processing logic, anddata acquisition systems). The tool 724 can comprise a downhole tool,such as an LWD tool or MWD tool. The wireline logging tool body 602 cancomprise a wireline logging tool, including a probe or sonde, forexample, coupled to a logging cable 614. Many embodiments can thus berealized.

Thus, a system 600, 700 can comprise a downhole tool body, such as awireline logging tool body 602 or a downhole tool 724 (e.g., an LWD orMWD tool body), and fiber optic cable 104 to provide signaling to theoptical detection system 100 or to components thereof (e.g., an opticaldetector 102) as described above.

The physical structure of such instructions can be operated on by one ormore processors. Executing instructions determined by these physicalstructures can cause the optical detection system 100 or componentsthereof to perform operations according to methods described herein. Theinstructions can include instructions to cause associated data or otherdata to be stored in a memory.

The wireline logging tool body 602 (FIG. 6) can include or otherwise beutilized in conjunction with any number of measurement tools such asresistivity tools, seismic tools, acoustic tools, temperature sensors,porosity sensors and others. In one embodiment, the wireline loggingtool body 602 is equipped with transmission equipment to communicateultimately to a surface processing unit of a surface logging facility616 (FIG. 6). Such transmission equipment can take any desired form, anddifferent transmission media and methods can be used. Examples ofconnections include wired, fiber optic, wireless connections and memorybased systems.

FIG. 8 is a diagram of a permanent monitoring system embodiment. In thesystem 800, cement 802 fills an annulus 804 formed radially betweencasing 723 and a borehole 612.

As used herein, the term “cement” is used to indicate a hardenablematerial which is used to seal off an annular space in a well, such asthe annulus 804. Cement is not necessarily cementitious, and other typesof materials (e.g., polymers, such as epoxies, etc.) can be used inplace of, or in addition to, a Portland type of cement. Cement canharden by hydrating, by passage of time, by application of heat, bycross-linking, and/or by any other technique.

As used herein, herein, the term “casing” is used to indicate agenerally tubular string that forms a protective wellbore lining. Casingmay include any of the types of materials know to those skilled in theart as casing, liner or tubing. Casing may be segmented or continuous,and may be supplied ready for installation, or may be formed in situ.

Conditions of cement, or any other downhole condition, can be monitoredusing the permanent monitoring system 800. One or more fiber opticcables 104 can be disposed in the well, so that they are operative tosense certain parameters in the well, for example properties of thecement 802. As depicted in FIG. 8, the fiber optic cable 104 ispositioned in the annulus 804 between the casing 723 and the borehole612, but in other examples the fiber optic cable 104 could be positionedin a wall of the casing, or adjacent the formation 714.

The fiber optic cable 104 can be strapped, clamped, or otherwise securedto an exterior of the casing 723. The system 800 further includescomponents of the system 100 (FIG. 1) for example optical detectionapparatus 101, measuring device 118 or any other component of the system100. Accordingly, the system 800 can monitor downhole conditions formonths or even years after well completion.

Additional Embodiments

As mentioned earlier herein with respect to FIG. 1, optical detectors102 in the optical detection system 100 (FIG. 1) can be SPDs. These SPDscan detect very low levels of light found in many photonics-basedapplications. Low light levels may, in some limit the ability to conductdownhole sensing operations. For example, some systems have losses of upto 110 dB, which limits the range, data rate, or the resolution ofoperation of optical detection systems. As further described earlierherein, the SNR in some systems can be reduced due to the presence ofnoise sources, including thermal noise and other noise. Some embodimentsprovide optical detection systems that use SPDs to enhance or increaseSNRs by removing noise sources and reducing signal loss. Someembodiments also provide more robust data detection for enhancedsensing.

SPDs, such as superconducting nanowire SPDs (SNSPDs), operate bydetecting a quantum state disturbance by an incoming photon incident onthe corresponding optical detector 102. By combining the use of SPD-typeoptical detector 102 at the surface with downhole units 116, verylow-energy signals (e.g., at the energy of a single photon) can bedetected. In addition, SNSPD-type optical detectors 102 can provideefficient operation at a wide range of wavelengths (e.g., fromultra-violet to mid-infrared wavelength regions), low dark counts (dueto the removal of thermal noise from the optical detection system 100),short recovery periods (e.g., recovery periods on the order of 1-10nanoseconds), and low timing jitter (e.g., timing jitter on the order of100-500 picoseconds). SPD-type optical detectors 102 can be integratedinto closed-cycle refrigerator-based detector systems, allowing foradvanced photon counting in oil and gas exploration operations that relyon portability and durability in fielded detection devices and systems.

SPDs can operate over wavelengths of 100-200 nm centered around a centerfrequency that depends on the fabrication of the SPD. By changingparameters of the fabrication (e.g., the timing or duration of etchingor the thickness of a nanowire layer, substrate, or other thickness),the center frequency can be constructed to span wavelengths betweenultraviolet wavelengths to mid/far infrared (˜200 nm to ˜10 micron).Once fabricated though, the SNSPD device spans up to ˜200 nm.Accordingly, to detect wavelengths covering a range of, e.g., 200 nm to10,000 nm, an optical detection apparatus should include 50 SPDs withcenter frequencies separated by 200 nm.

FIG. 9 a block diagram of an optical detection system 900 including anSPD in accordance with various embodiments. An optical detectionapparatus 904 includes an optical detector (e.g., an SNSPD) 902 fordetecting light received at an input section 906 of fiber optic cable908. The optical detection apparatus 904 can further include a housing910 for enclosing the optical detector 902 and optically shielding theoptical detector 902. In some embodiments, the input section 906 offiber optic cable can pass through a housing aperture to permit passageof the fiber optic cable 908. However, embodiments are not limitedthereto, and in some embodiments, a coupler can be mounted so that thefiber optic cable 908 terminates at a boundary of the housing 910.

Similarly to the optical detection system 100 (FIG. 1), the opticaldetection system 900 includes a light source 912, separate from theoptical detector 902 and the housing 910, for providing light throughthe fiber optic cable 908 to a downhole sensing unit 914. The lightsource 912 may be provided at a surface of the Earth, for example.

The optical detection system 900 further includes a cryogenic cooler 916to remove heat to maintain the temperature of a light-sensitive regionof the optical detector 902 within a superconducting temperature rangeof the optical detector 902. As described above regarding the coolingmechanism 112 (FIG. 1), in some embodiments, the cryogenic cooler 916operates using one of liquid helium (He) and liquid nitrogen (N₂), toreach temperatures ranging down to about 77 K or about 2.5 K. Thecryogenic cooler 916 can operate in a closed-loop system with a fewhundred watts (e.g. 100-200 watts) of power, and has low servicing andreplenishment specifications, which enables portable and low-risk use inoil and gas operations. As described above, the cryogenic cooler 916 canbe of one or more of a variety of configurations, includingDilution-Magnetic, Collins-Helium Liquefier, Joule-Thomson,Stirling-cycle cryocooler, self-regulated Joule-Thomson, Closed-CycleSplit-Type Stirling, Pulse Tube, a two-stage Gifford-McMahon cryogeniccooler or multi-stage Gifford-McMahon cryogenic cooler, or a coolerusing magnetocaloric effect, by way of example.

Similarly to the optical detection system 100 (FIG. 1), the opticaldetection system 900 can further include multiple optical detectors,which can be SPD optical detectors, SNSPD optical detectors, or othertypes of non-SPD optical detectors. An SPD optical detector 902 candetect light within a first dynamic range while non-SPD opticaldetectors can detect levels of light having a dynamic range distinct,but not necessarily exclusive, of the first dynamic range.

An SNSPD for use in various embodiments can be configured to include asuperconductive meandering nanowire structure. FIG. 10A illustrates anSNSPD 1000 using meandering nanowires 1002 in accordance with variousembodiments. FIG. 10B illustrates an enlarged view of the SNSPD 1000 ofFIG. 10A. The SNSPD 1000 of FIGS. 10A and 10B can include niobiumnitride nanowires grown on magnesium oxide or sapphire substrates,although embodiments are not limited to any particular substratematerial or nanowire material. For example, the nanowires 1002 can alsoinclude tungsten silicide, niobium silicide, and tantalum nitride. Thenanowires 1002 meander between bonding pads 1004 in a compact meandergeometry to create a square or circular pixel with high detectionefficiency. Embodiments are not limited to a meandering nanowire 1002.For example, the nanowires 1002 can be configured in a superconductiveinterleaved nanowire structure, and/or nanowires 1002 can be parallel toeach other.

The dimensions (e.g., diameter, length, etc.) of the nanowires 1002 arechosen such that a uniform optical cavity, optimized for the specificwavelength of the produced light, is provided along the length ofnanowire. The nanowires 1002 can be fabricated such that the diameter ofthe nanowire 1002 is sufficiently wide to capture the desired light. Forexample, the diameter of the nanowire 1002 should be larger thanκ/2n_(w), wherein λ is the wavelength of the desired light and n_(w) isthe refractive index of the nanowire. For example, nanowires 1002 usedin various embodiments can have diameters of about 90-100 nanometers orless, or to a few hundred nanometers.

Some embodiments provide a reflective layer or a cavity on the substratethat extends under the nanowire 1002. The reflective layer can reflectlight that is guided by the nanowire 1002. The reflective layer can beprovided in the form of a multilayered structure comprising repeatedlayers of silicates for example, or as a metal film to provide furthersystem efficiencies due to light or photon reflection that allows thenanowire a second chance to detect a given photon or photons. Someembodiments can include many layers (e.g., three to five layers, ormore) of photonic crystals (e.g., Ta₂O₅ or SiO₂),

Referring again to FIG. 9, a cooling mechanism (e.g., cryogenic cooler916 (or a cooling mechanism 112 (FIG. 1) or a cooling mechanism 312(FIG. 3) can be used to cool a nanowire of the optical detector 902below the nanowire superconducting critical temperature. A power source918 provides the optical detector 902 with current. A photon incident ona nanowire of the optical detector 902 breaks Cooper pairs and creates alocalized non-superconducting region, or hotspot, with finite electricalresistance on the nanowire. A monitoring unit 920 coupled to an output922 of the optical detector 902 has an impedance lower than an impedanceof a non-superconducting region of the optical detector 902 so thatcurrent is shunted to the monitoring unit 920, resulting in a measurablevoltage at the monitoring unit 920. When the current is shunted from theoptical detector 902, the non-superconducting region of the opticaldetector 902 can then be re-cooled below a superconducting criticaltemperature. The monitoring unit 920 detects that a signal has beenreceived at the optical detector 902 and the monitoring unit 920 canprovide the signal, or notification of the signal, for furtherprocessing. The monitoring unit 920 can be included as a component ofthe system 600, 700, 800 (FIG. 6, FIG. 7, and FIG. 8 respectively) fordisplay or any other use in determining properties of a downholeformation. The monitoring unit 920 can also include, or be coupled toother components that include, a timing unit and a counting unit todetermine when photons hit the optical detector 902, and how manyphotons hit the optical detector 902. A timing unit 921 can provide atime stamp. The monitoring unit 920 can also provide a counter to countthe protons in reflected measurement signal or other signal.

In some embodiments, an SNSPD-type optical detector 902 includesmultiple superconducting nanowire structures. FIG. 11 illustrates anSNSPD having multiple superconducting nanowire structures 1102, 1104,1106, 1108, 1110, 1114, 1116, 1118 arranged in parallel in accordancewith various embodiments. In some embodiments, the multiplesuperconducting nanowire structures 1102, 1104, 1106, 1108, 1110, 1114,1116, 1118 share a common ground 1120, or groups of multiplesuperconducting nanowire structures share a common ground. In someembodiments, the common ground is on a second surface of the magnesiumoxide substrate (or other type of substrate) opposing the first surfaceof the magnesium oxide substrate on which the superconducting nanowirestructures 1102, 1104, 1106, 1108, 1110, 1114, 1116, 1118 are grown. Insome embodiments, at least two of the multiple superconducting nanowirestructures have separate power sources. In some embodiments, themultiple superconducting nanowire structures 1102, 1104, 1106, 1108,1110, 1114, 1116, 1118 share are each configured to measure one signal(e.g., a Stokes signal or an anti-Stokes signal, as described in moredetail below). In another array of pixels, on the same chip but in adifferent location on the chip in some embodiments, superconductingnanowire structures will measure the Stokes or anti-Stokes signal thatis the counterpart of the Stokes or anti-Stokes signal measured by themultiple superconducting nanowire structures 1102, 1104, 1106, 1108,1110, 1114, 1116, 1118.

Referring again to FIG. 9, by adjusting or adapting the width and pitchof the nanowire/s during fabrication of the optical detector 902, theoptical detector 902 can be made more efficient. In a 100% efficientSPD-type optical detector 902, a signal is produced every time a singlephoton enters optical detector 902. However, photons can arrive at theoptical detector 902 at different polarizations (among othervariations). SNSPDs (in contrast to other types of optical detectors)are sensitive to the polarization of light due to the usage of nanowiresin their construction, because nanowires are relatively straight alongtheir length, and thus nanowires detect fields that align with thenanowire along its length.

Multilayer SNSPD constructions can overcome these and other challengesto provide improved efficiency. For example, as illustrated in FIG. 12,two or more layers 1202 and 1204 can be vertical stacked and connectedin parallel (with a common power source, common ground, etc.) to form amultilayer SNSPD 1200. Each layer 1202, 1204 can have nanowires 1206,1208 meandering in different patterns, or oriented at orthogonal angleswith respect to one another, so that reduced absorptance of light 1210incident on the SNSPD 1200 at one layer 1202, 1204 will be offset oradjusted for by absorptance variations at the other layer 1202, 1204. Byusing multiple layers of nanowires 1206, 1208 with different pitches andpatterns, polarization sensitivity can be reduced and the efficiency ofthe optical detector 902 (FIG. 9) can be enhanced or improved.Additionally, or in the alternative, some embodiments provide multiplelayers, each layer detecting a different wavelength or range ofwavelengths, so that the optical detector 902 can detect a larger rangeof wavelengths.

As with other embodiments discussed earlier herein with respect to FIGS.1-5, the optical detector 902 can be used in a variety of opticalsystems, including various types of sensor systems. In sensor-basedapplications, use of SPD-type optical detectors can improve theresolution or deployable distance of distributed acoustic sensing (DAS)systems, distributed temperature sensing (DTS) systems, distributedstrain systems (DSS), and distributed chemical sensing systems, etc. Useof SPD-type optical detectors can also allow for high transmission lossinterconnects, enabling offshore monitoring. Optical detection systemsthat use SPD-type optical detectors can be used in systems includingmany low transmission loss interconnects, for example drill strings thathave an interconnect, and some transmission loss, at every joint.Furthermore, optical detection systems that make use of SPD-type opticaldetectors can detect visible light, and therefore such systems can findextended usage in spectroscopy applications, in rubidium-basedgyroscopes, and in rubidium-based magnetometers in on-shore and offshoredrilling and oil exploration applications. Optical detection systemsthat make use of SPD-type optical detectors can detect very weak signals(e.g., light signals of one photon can be detected), and therefore theseoptical detection systems can perform operations to operational depthsof 35,000 feet or more. Such systems can also operate with minimal or nodownhole electronics, leading to cost savings for operators.

FIG. 13 is a flow chart of an example method 1300 of optical sensingusing an SNSPD optical detector in accordance with some embodiments. Theexample method 1300 can be performed by the optical detection system 900(FIG. 9) or by components thereof. The example method 1300 begins atoperation 1302 by coupling an optical detection apparatus 904 to adownhole sensing unit 914 through a fiber optic cable 908. The opticaldetection apparatus 904 includes at least one SNSPD-type opticaldetector 902.

The example method 1300 continues with operation 1304 with the cryogeniccooler 916 maintaining the temperature of the SNSPD-type opticaldetector 902 within a superconducting range of the SNSPD-type opticaldetector 902.

The example method 1300 continues with operation 1306 with the powersource 918 providing current to the SNSPD-type optical detector 902.

The example method 1300 continues with operation 1308 with detecting anoptical signal at the optical detection apparatus 904 upon themonitoring unit 920 measuring a voltage proportional to thesuperconducting critical current of the SNSPD-type optical detector 902.The voltage will be generated when photons received at the SNSPD-typeoptical detector 902 raise the temperature of the SNSPD-type opticaldetector 902 above the superconducting range of the SNSPD-type opticaldetector 902 to create an electrical impedance in the SNSPD-type opticaldetector 902. The optical signal can be time-correlated to determine howmany photons were detected in a certain time period. Other operationscan include counting the number of photons received during a time periodto provide a photon count and detecting a downhole property based on thephoton count.

In some embodiments in which the optical detection apparatus 904 furtherincludes at least one optical detector 902 that is not an SNSPD or SPD,the example method 1300 further includes directing optical signalshaving a power level greater than a threshold power level to the atleast one optical detector that is not an SPD or SNSPD. In embodimentsin which the downhole sensing unit 914 includes an intrinsic fiber opticsensor, the example method 1300 can include providing an optical signalto the intrinsic fiber optic sensor; and receiving a reflected opticalsignal, responsive to providing the optical signal, that represents atleast one downhole property.

Arrayed DAS Using Single-Photon Detectors

As mentioned earlier herein with respect to FIGS. 1-2, optical detectors102 can include groups of optical detectors 204 (FIG. 2) dedicated toDAS. DAS operates using backscattered Rayleigh signals. However,Rayleigh signals can experience significant loss in intensity beforereaching surface systems and, accordingly, DAS operations may be limitedin resolution, length of operation, and signal quality.

Furthermore, DAS systems can be costly in that they make use ofdedicated laser systems and optical receivers. Embodiments provideapparatuses and methods that use fewer laser sources and fewer arrayedSPDs (e.g., one source and one SPD) to operate more than one DAS systemsimultaneously. Furthermore, embodiments provide for arrayed SPDs to bearranged to form on-chip balanced photodetectors for improved speed inoperation and reduction in DAS system cost.

FIG. 14 is a block diagram of an arrayed DAS system 1400 in accordancewith various embodiments. A single laser pulse is generated by a source1402 and passes through optics (e.g., variable attenuators, pumpfilters, erbium doped fiber amplifiers, couplers, and other opticaldevices) 1404. The source 1402 and optics 1404 can be housed in asurface system, for example, the source 1402 and optics 1404 can becollocated with the measuring device 118 and display 120 (FIGS. 6, 7 and8). The laser pulse is separated onto one path 1406, 1410, 1414, 1418for each DAS system 1408, 1412, 1416, and 1420, where a DAS system 1408,1412, 1416, 1420 includes a fiber optic cable 1424, 1426, 1428, 1430, acirculator 1442, 1444, 1446, 1448 and optics 1432, 1434, 1436, 1438.Because the SPD array 1422 can detect faint signals (e.g., signalsconsisting of as few as one photon), a single source 2402 can have itspower divided among several light paths (by a division circuit,splitter, or other circuitry) and still allow operation of DAS systemsaccording to operational requirements. Furthermore, the use of signalswith lower power levels allow the laser pulse produced by the source1402 to maintain linearity during propagation, reducing nonlineareffects, such as self-phase modulation, cross-phase modulation,four-wave mixing, or exponential growth of the backscattering signal.Additionally, two laser sources 1302 at different wavelengths can beused as well.

In embodiments, light (e.g., a “measurement signal”) provided by thesource 1402 travels through circulators 1442, 1444, 1446 and 1448 toeach fiber optic cable 1424, 1426, 1428 and 1430 (also shown in FIG. 7)being used for DAS. Each fiber optic cable 1424, 1426, 1428 and 1430 caninclude at least one sensing location. The sensing locations generatereflected measurement signals (e.g., backscattered Rayleigh signals)representative of measurement parameters at corresponding positionsdownhole. In some embodiments, the sensing locations can be locations ofspontaneous Rayleigh scattering. The backscattered Rayleigh signal/sreturn through conditioning optics circuitry 1432, 1434, 1436 and 1438(such as, e.g., wavelength demuliplexers, couplers, or optical filters)before being detected by the SPD array 1422. The backscattered Rayleighsignal/s are typically at about the same optical frequency as the lightprovided by the source 1402. As shown in FIGS. 6 and 7, the SPD array1422 can be located in a surface system; for example, the SPD array 1422can be collocated with the measuring device 118 and the display 120.

In embodiments, the fiber optic cables 1424, 1426, 1428 and 1430 can beshared with DTS systems. DTS systems use inelastic (Raman) scatteringrather than the elastic (Rayleigh) scattering being employed by DASsystems. As DTS is often performed at a shorter wavelength than DAS(e.g., 1064 nm versus 1550 nm), a DAS system and DTS system can operatein parallel without causing optical interference. A wavelength divisionmultiplexer can be provided to couple light sources for both DTS and DASto the sensing fiber. If some loss is acceptable, the multiplexer can bereplaced with a beam splitter.

The SPD array 1422 can be matched so that one element of the SPD array1422 detects one signal and a second element detects a different signal.The use of matched detectors can enable improved use of available lightand can cancel light common to both outputs, in particular common-modenoise. The SPD array 1422 outputs can be compared in a balanced receiver1439 configuration (using either external electronic circuitry or anon-chip integrated system, for example, as described in more detailbelow with reference to FIG. 16) to retrieve phase information for anyor all of the light signals. In some examples, the phase informationincludes in-phase (I) or quadrature (Q) phase information. Processingcircuitry 1440 can then produce an acoustic signal representative of adownhole property based on the phase information (e.g., the I/Q phaseinformation) of the backscattered Rayleigh signal using methodologiesand algorithms as described later herein.

FIG. 15A illustrates an example SPD array 1422 for fiber imaging inaccordance with various embodiments. Arrays of SNSPDs (e.g., “pixels”)1502 can be used to provide DAS under low-light (e.g., “photon starved”)conditions. The array 1422 shown in FIG. 15A includes 64 pixels 1502. Incomparing FIG. 15A with FIG. 11, it will be appreciated that each ofnanowire structures 1102, 1104, 1106, 1108, 1110, 1114, 1116, 1118correspond to pixels 1502. The pixels 1502 can be fabricated on asilicon substrate with a thermally-oxidized silicon oxide layer or onsingle-crystal MgO, although embodiments are not limited thereto.Interconnection lines 1504 can be formed in the spaces between pixels1502 and these interconnection lines can be connected to coplanarwaveguide lines 1506. By creating a regular pattern over a large numberof pixels (e.g., 64-100 pixels, though as few as two pixels can be used,or more than 100 pixels can be used), operators can gain informationabout the spatial distribution of the incoming stream of photons.

FIG. 15B illustrates an example grid 1510 of multiple SNSPD arrays 1512.Each of the multiple SNSPD arrays 1512 can couple to at least one fiberoptic cable 1424, 1426, 1428, 1430 (FIG. 14, not shown in FIG. 15B). Insome embodiments, the grid 1510 will be comprised of a square number ofSNSPD arrays 1512, for example, the grid 1510 can comprise fifteen SNSPDarrays 1512 in one row 1514 of the grid 1510 and fifteen columns 1516 ofSNSPD arrays 1512. The number of SNSPD arrays 1512 can be selected basedon known, detected or predicted properties of the backscattered Rayleighsignals, for example, the number of SNSPD arrays 1512 can be selectedbased on power, wavelength, polarization, etc. of the backscatteredRayleigh signals that are expected to be received at the grid 1510.

FIG. 16 is a block diagram of a balanced receiver 1439 circuitry forretrieving phase information from a backscattered Rayleigh signal inaccordance with various embodiments. The balanced receiver 1439 canemploy either heterodyne or homodyne coherent detection to extract phaseinformation from the backscattered Rayleigh signal. Description of thebalanced receiver 1439 is made with reference to some of the elements ofFIG. 14.

The balanced receiver 1439 includes at least two inputs 1602 and 1604.One of the inputs (e.g., 1602) can include the Rayleigh scattering ofthe probe pulse, wherein the probe pulse was provided by the source 1402(FIG. 14).

Generally, if backscattered Rayleigh signals are sent directly to adetector, the phase information is lost and the signal relates purely tothe amplitude of the backscattered Rayleigh signal. However, when thebackscattered Rayleigh signal is demodulated through mixing with lightfrom a surface source (e.g., a local oscillator (LO) source), then thebackscattered Rayleigh signal and the LO source combine to provide asignal that retains phase information of the backscattered Rayleighsignal. Accordingly, in some embodiments, the balanced receiver includesan LO 1612 and at least one of the two inputs 1602 and 1604 can beconfigured to receive an output of the LO 1612. Such a system can bereferred to as a coherent-detection system.

If the LO 1612 and the backscattered Rayleigh signals are at differentfrequencies, the system is referred to as a heterodyne system; otherwisethe system is referred to as a homodyne system. In homodyne systems, therelative phase of the backscattered Rayleigh signal and LO 1612 signalis zero (it is usually non-zero for heterodyne), so homodyne systems aremore phase-sensitive than heterodyne systems. Furthermore, coherenthomodyne detection methods and systems include at least one fewer mixingstages than heterodyne methods and systems, thus adding lower noiseduring the demodulation process, compared with heterodyne demodulation.Heterodyne detection, however, allows flexibility in changing theoptical carrier frequency if the receiver is not co-located with thetransmitter.

The inputs 1602 and 1604 can be split into two beams by splitters 1606and 1608 to provide polarization components 1610 of the backscatteredRayleigh signal and polarization components 1612 of the LO signal. Thebalanced receiver 1439 can include 90° optical hybrid mixers 1614 and1616 to receive the polarization components 1610 of the backscatteredRayleigh signal and polarization components 1612 of the LO signal. Thesemixers 1614 and 1616 provide I and Q signals allowing determination ofthe amplitude and phase of the received backscattered Rayleigh signal.The beats between the signal polarization components 1610 and 1612 aredetected by photodiodes 1618, and the resulting photocurrents areamplified and converted to output voltages I_(x), Q_(x), I_(y) and Q_(y)by linear trans-impedance amplifiers (TIA) 1620.

Data acquisition circuitry 1622 processes output voltages I_(x), Q_(x),I_(y) and Q_(y) to determine and track phase at each separate locationto generate an acoustic signal. Systems and methods in accordance withvarious embodiments generate an acoustic signal by first finding thephase (ϕ)=tan⁻¹ (Q/I) at each separate location x. For each x, systemsfind the change in phase with time (dϕ/dt) and then take the Fouriertransform to get the acoustic spectrum. The total acoustic power wouldbe the integral of the acoustic spectrum. The acoustic spectrum will berepresentative of a downhole property and can be processed according toknown methods to determine downhole properties. Data acquisition outputscan be provided to the processor 1440 for other processing, for storagein local or remote systems, and for processing for display on a userinterface, among other uses. The data acquisition circuitry 1622 caninclude a suitable processor (e.g., general purpose processor,microcontroller) and associated memory device for performing processingfunctions, such as normalization of the acquired data, data averaging,data storage, and/or display to a user or operator of the system.

FIG. 17 is a flow chart of an example method 1700 of measuring adownhole property in accordance with various embodiments. The examplemethod 1700 can be performed by the arrayed DAS system 1400 (FIG. 14),or by components thereof. The example method 1700 begins with operation1702 with disposing a fiber optic cable (e.g., fiber optic cable/s 1424,1426, 1428 and/or 1430, FIG. 14) in a downhole location. The fiber opticcable 1424, 1426, 1428 and/or 1430 can include a sensing locationconfigured to generate backscattered Rayleigh signals.

The example method 1700 continues with operation 1704 with the source1402 transmitting a measurement signal into the fiber optic cable/s1424, 1426, 1428 and/or 1430 from a surface location. The measurementsignal can have an energy level less than or equal to a saturationenergy level of a superconducting nanowire single-photon detector(SNSPD) coupled to the fiber optic cable/s 1424, 1426, 1428 and/or 1430.The sensing locations generate reflected measurement signals (e.g.,backscattered Rayleigh signals). The measurement signal can includelight having a wavelength within a visible or infrared range. Inembodiments, the source 1402 can provide the measurement signal inpulses of fewer than 100 nanoseconds (and in some embodiments fewer than5 nanoseconds) in length.

The example method 1700 continues with operation 1706 with circuitry(e.g., data acquisition circuitry 1622, processor 1440, etc.) generatingan acoustic signal representative of a downhole property based the phaseof the backscattered Rayleigh signal. In examples, the circuitry cangenerate the acoustic signal by interfering (using an interferometer)the backscattered Rayleigh signal with a coherent signal. In someembodiments, the phase includes an I component and a Q component.

In some embodiments, at least one fiber optic cable/s 1424, 1426, 1428and/or 1430 can provide reflected measurement signals (e.g.,backscattered Rayleigh signals, or other light signals, etc.) to non-SPDdetectors, or to SPD detectors that are not SNSPD detectors (e.g.,photomultiplier tubes, avalanche photodiodes, frequency up-conversiondetectors, visible light photon counters, transition edge sensors,quantum dots, etc.) The distance between the source and the sensinglocation can be 100 feet or more. In embodiments, backscattered Rayleighsignals (or any other signals reflected from downhole sensing locations)can be routed based on one or more properties such as polarization,wavelength, and the number of photons in the reflected signal (e.g., inthe backscattered Rayleigh signal).

In various embodiments, a non-transitory machine-readable storage devicecan comprise instructions stored thereon, which, when performed by amachine, cause the machine to perform operations, the operationscomprising one or more features similar to or identical to features ofmethods and techniques described herein. A machine-readable storagedevice, herein, is a physical device that stores data represented byphysical structure within the device. Examples of machine-readablestorage devices can include, but are not limited to, memory in the formof read only memory (ROM), random access memory (RAM), a magnetic diskstorage device, an optical storage device, a flash memory, and otherelectronic, magnetic, or optical memory devices, including combinationsthereof. These can be provided in integrated chips that include opticaldetectors 102, or in other surface computer systems for takingmeasurements or analyzing measurements as part of the optical detectionsystem 100, 300, 900, and 1400 (FIGS. 1, 3, 9, and 14).

Any of the above components, for example components of the opticaldetection system 100, 300, 900, and 1400, can all be characterized as“modules” herein. Such modules can include hardware circuitry, and/or aprocessor and/or memory circuits, software program modules and objects,and/or firmware, and combinations thereof, as desired by the architectof the optical detection system 100 as appropriate for particularimplementations of various embodiments. For example, in someembodiments, such modules can be included in an apparatus and/or systemoperation simulation package, such as a software electrical signalsimulation package, a power usage and distribution simulation package, apower/heat dissipation simulation package, and/or a combination ofsoftware and hardware used to simulate the operation of variouspotential embodiments.

It should also be understood that the apparatus and systems of variousembodiments can be used in applications other than for loggingoperations, and thus, various embodiments are not to be so limited. Theillustrations of optical detection systems 100, 300, 900, and 1400 areintended to provide a general understanding of the structure of variousembodiments, and they are not intended to serve as a completedescription of all the elements and features of apparatus and systemsthat might make use of the structures described herein.

Applications that can include the novel apparatus and systems of variousembodiments include electronic circuitry used in high-speed computers,communication and signal processing circuitry, modems, processormodules, embedded processors, data switches, and application-specificmodules. Some embodiments include a number of methods.

It should be noted that the methods described herein do not have to beexecuted in the order described, or in any particular order. Moreover,various activities described with respect to the methods identifiedherein can be executed in iterative, serial, or parallel fashion.Information, including parameters, commands, operands, and other data,can be sent and received in the form of one or more carrier waves.

Upon reading and comprehending the content of this disclosure, one ofordinary skill in the art will understand the manner in which a softwareprogram can be launched from a computer-readable medium in acomputer-based system to execute the functions defined in the softwareprogram. One of ordinary skill in the art will further understand thevarious programming languages that can be employed to create one or moresoftware programs designed to implement and perform the methodsdisclosed herein. For example, the programs can be structured in anobject-orientated format using an object-oriented language such as Javaor C#. In another example, the programs can be structured in aprocedure-orientated format using a procedural language, such asassembly or C. The software components can communicate using any of anumber of mechanisms well known to those skilled in the art, such asapplication program interfaces or interprocess communication techniques,including remote procedure calls. The teachings of various embodimentsare not limited to any particular programming language or environment.Thus, other embodiments can be realized.

In summary, using the apparatus, systems, and methods disclosed hereincan provide more accurate measurements by optical detection apparatusesthrough removal or reduction of noise sources including thermal noisesources. These advantages can significantly enhance the value of theservices provided by an operation/exploration company, while at the sametime controlling time-related costs.

The accompanying drawings that form a part hereof, show by way ofillustration, and not of limitation, specific embodiments in which thesubject matter can be practiced. The embodiments illustrated aredescribed in sufficient detail to enable those skilled in the art topractice the teachings disclosed herein. Other embodiments can beutilized and derived therefrom, such that structural and logicalsubstitutions and changes can be made without departing from the scopeof this disclosure. This Detailed Description, therefore, is not to betaken in a limiting sense, and the scope of various embodiments isdefined only by the appended claims, along with the full range ofequivalents to which such claims are entitled.

Such embodiments of the inventive subject matter can be referred toherein, individually and/or collectively, by the term “invention” merelyfor convenience and without intending to voluntarily limit the scope ofthis application to any single invention or inventive concept if morethan one is in fact disclosed. Thus, although specific embodiments havebeen illustrated and described herein, it should be appreciated that anyarrangement calculated to achieve the same purpose can be substitutedfor the specific embodiments shown. This disclosure is intended to coverany and all adaptations or variations of various embodiments.Combinations of the above embodiments, and other embodiments notspecifically described herein, will be apparent to those of skill in theart upon reviewing the above description.

Various examples include:

Example 1 is a system, detection system or other device or apparatus(e.g., an optical detection system), comprising: a fiber optic cable tobe disposed in a downhole location, the fiber optic cable including asensing location to generate a backscattered Rayleigh signalrepresentative of measurement parameters; a light source, at a surfaceof the Earth, to transmit a measurement signal to cause the sensinglocation to provide the backscattered Rayleigh signal; an opticaldetector device comprising at least one single-photon detector (SPD) fordetecting the backscattered Rayleigh signal received over the fiberoptic cable; and circuitry to produce an acoustic signal representativeof a downhole property based on a phase of the backscattered Rayleighsignal.

In Example 2, the subject matter of Example 1 can optionally includewherein the phase includes an in-phase (I) component and quadrature (Q)component.

In Example 3, the subject matter of Example 2 can optionally includewherein the circuitry includes at least two inputs, at least one of theat least two inputs to receive the backscattered Rayleigh signal; mixercircuitry to receive the two inputs; and circuitry to interfere outputsof the mixer circuitry and to determine the I and Q components of thebackscattered Rayleigh signal.

In Example 4, the subject matter of Example 3 can optionally includewherein the circuitry includes an oscillator, and wherein at least oneof the at least two inputs is configured to receive an output of theoscillator.

In Example 5, the subject matter of Example 2 can optionally include anoptical splitter to split the backscattered Rayleigh signal into aplurality of optical paths, and wherein the optical detector includes aplurality of SPDs, the plurality of SPDs being arranged in pairs suchthat a first SPD of a pair detects a first light signal on a firstoptical path of the plurality of optical paths, and a second SPD of thepair detects a second light signal on a second optical path of theplurality of paths.

In Example 6, the subject matter of Example 5 can optionally includewherein the plurality of SPDs include superconducting nanowire SPDs(SNSPDs), and wherein the optical detection system further comprises acryogenic cooler configured to maintain the temperature oflight-sensitive regions of the SNSPDs within a superconductingtemperature range of the SNSPDs.

In Example 7, the subject matter of Example 6 can optionally includewherein the cryogenic cooler operates using one of liquid helium (He)and liquid nitrogen (N₂.).

In Example 8, the subject matter of Example 2 can optionally include arouting circuit to route the backscattered Rayleigh signal to provide atleast a second backscattered Rayleigh signal when a count of the numberof photons in the backscattered Rayleigh signal reaches or exceeds athreshold number of photons.

In Example 9, the subject matter of Example 2 can optionally includewherein the fiber optic cable includes a plurality of sensing locationsalong a measurement length of the fiber optic cable, the plurality ofsensing locations configured to provide a plurality of backscatteredRayleigh signals representative of values of the measurement parametersat corresponding positions downhole.

In Example 10, the subject matter of Example 9 can optionally includewherein the plurality of sensing locations are locations of spontaneousRayleigh scattering.

In Example 11, the subject matter of Example 2 can optionally includewherein the plurality of SPDs share a common ground.

In Example 12, the subject matter of Example 2 can optionally includewherein the fiber optic cable is configured for use as a distributedacoustic sensing (DAS) fiber/

In Example 13, the subject matter of Example 12 can optionally include aplurality of fiber optic cables, each of the plurality of fiber opticcables configured to operate as a DAS fiber.

In Example 14, the subject matter of Example 2 can optionally include amonitoring unit coupled to an output of the SNSPD, the monitoring unithaving an impedance lower than an impedance of a non-superconductingregion of the SNSPD, the monitoring unit configured to providemeasurement data of the SNSPD; and a display to display a graphicalrepresentation of the measurement data.

In Example 15, the subject matter of Example 2 can optionally includewherein the light source is configured to transmit a measurement signalhaving an energy level below a threshold energy level such that eachbackscattered Rayleigh signal has an energy below a saturation energylevel of each of the plurality of SPDs.

In Example 16, the subject matter of Example 2 can optionally include ahousing for enclosing the optical detector and to optically shield theoptical detector, the housing including an aperture for passage of thefiber optic cable.

Example 17 is a method for estimating a downhole property, the methodcomprising: disposing a fiber optic cable in a downhole location, thefiber optic cable including a sensing location configured to generatebackscattered Rayleigh signals; transmitting a measurement signal intothe fiber optic cable from a surface location, the measurement signalhaving an energy level less than or equal to a saturation energy levelof a superconducting nanowire single-photon detector (SNSPD) coupled tothe fiber optic cable, to cause the sensing location to return thebackscattered Rayleigh signal to the surface location; and generating anacoustic signal representative of a downhole property based on a phaseof the backscattered Rayleigh signal by interfering the backscatteredRayleigh signal with a coherent signal.

In Example 18, the subject matter of Example 17 can optionally includewherein the phase includes an in-phase (I) component and quadrature (Q)component.

In Example 19, the subject matter of Example 18 can optionally includesplitting the backscattered Rayleigh signal into a plurality of opticalpaths; detecting a first light signal on a first optical path of theplurality of optical paths; and detecting a second light signal on asecond optical path of the plurality of paths.

In Example 20, the subject matter of Example 18 can optionally includemaintaining the temperature of light-sensitive regions of the SNSPDswithin a superconducting temperature range of the SNSPDs.

In Example 21, the subject matter of Example 18 can optionally includerouting the backscattered Rayleigh signal to provide at least a secondbackscattered Rayleigh signal based on at least one of polarization,wavelength, and a count of the number of photons in the backscatteredRayleigh signal.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose canbe substituted for the specific embodiments shown. Various embodimentsuse permutations or combinations of embodiments described herein. It isto be understood that the above description is intended to beillustrative, and not restrictive, and that the phraseology orterminology employed herein is for the purpose of description.Combinations of the above embodiments and other embodiments will beapparent to those of ordinary skill in the art upon studying the abovedescription.

What is claimed is:
 1. An optical detection system comprising: a fiberoptic cable to be disposed in a downhole location, the fiber optic cableincluding a sensing location to generate a backscattered Rayleigh signalrepresentative of measurement parameters; a light source, at a surfaceof the Earth, to transmit a measurement signal to cause the sensinglocation to provide the backscattered Rayleigh signal; an opticaldetector device comprising at least one single-photon detector (SPD) fordetecting the backscattered Rayleigh signal received over the fiberoptic cable; circuitry to produce an acoustic signal representative of adownhole property based on a phase of the backscattered Rayleigh signal,wherein the phase includes an in-phase (I) component and quadrature (Q)component; and an optical splitter to split the backscattered Rayleighsignal into a plurality of optical paths, and wherein the opticaldetector includes a plurality of SPDs, the plurality of SPDs beingarranged in pairs such that a first SPD of a pair detects a first lightsignal on a first optical path of the plurality of optical paths, and asecond SPD of the pair detects a second light signal on a second opticalpath of the plurality of paths.
 2. The optical detection system of claim1, wherein the circuitry includes: at least two inputs, at least one ofthe at least two inputs to receive the backscattered Rayleigh signal;mixer circuitry to receive the two inputs; and circuitry to interfereoutputs of the mixer circuitry and to determine the I and Q componentsof the backscattered Rayleigh signal.
 3. The optical detection system ofclaim 2, wherein the circuitry includes an oscillator, and wherein atleast one of the at least two inputs is configured to receive an outputof the oscillator.
 4. The optical detection system of claim 1, whereinthe plurality of SPDs include superconducting nanowire SPDs (SNSPDs),and wherein the optical detection system further comprises a cryogeniccooler configured to maintain the temperature of light-sensitive regionsof the SNSPDs within a superconducting temperature range of the SNSPDs.5. The optical detection system of claim 4, wherein the cryogenic cooleroperates using one of liquid helium (He) and liquid nitrogen (N₂.). 6.The optical detection system of claim 1, wherein the fiber optic cableincludes a plurality of sensing locations along a measurement length ofthe fiber optic cable, the plurality of sensing locations configured toprovide a plurality of backscattered Rayleigh signals representative ofvalues of the measurement parameters at corresponding positionsdownhole.
 7. The optical detection system of claim 6, wherein theplurality of sensing locations are locations of spontaneous Rayleighscattering.
 8. The optical detection system of claim 1, wherein: theplurality of SPDs share a common ground; or the fiber optic cable isconfigured for use as a distributed acoustic sensing (DAS) fiber.
 9. Theoptical detection system of claim 8, further comprising a plurality offiber optic cables, each of the plurality of fiber optic cablesconfigured to operate as a DAS fiber.
 10. The optical detection systemof claim 1, further comprising: a housing for enclosing the opticaldetector and to optically shield the optical detector, the housingincluding an aperture for passage of the fiber optic cable.
 11. A methodfor measuring a downhole property, the method comprising: disposing afiber optic cable in a downhole location, the fiber optic cableincluding a sensing location configured to generate backscatteredRayleigh signals; transmitting a measurement signal into the fiber opticcable from a surface location, the measurement signal having an energylevel less than or equal to a saturation energy level of asuperconducting nanowire single-photon detector (SNSPD) coupled to thefiber optic cable, to cause the sensing location to return thebackscattered Rayleigh signal to the surface location; and generating anacoustic signal representative of a downhole property based on a phaseof the backscattered Rayleigh signal by interfering the backscatteredRayleigh signal with a coherent signal.
 12. The method of claim 11,wherein the phase includes an in-phase (I) component and quadrature (Q)component.
 13. The method of claim 12, further comprising: splitting thebackscattered Rayleigh signal into a plurality of optical paths;detecting a first light signal on a first optical path of the pluralityof optical paths; and detecting a second light signal on a secondoptical path of the plurality of paths.
 14. The method of claim 12,further comprising: maintaining the temperature of light-sensitiveregions of the SNSPDs within a superconducting temperature range of theSNSPDs.
 15. The method of claim 12, further comprising: routing thebackscattered Rayleigh signal to provide at least a second backscatteredRayleigh signal based on at least one of polarization, wavelength, and acount of the number of photons in the backscattered Rayleigh signal. 16.An optical detection system comprising: a fiber optic cable to bedisposed in a downhole location, the fiber optic cable including asensing location to generate a backscattered Rayleigh signalrepresentative of measurement parameters; a light source, at a surfaceof the Earth, to transmit a measurement signal to cause the sensinglocation to provide the backscattered Rayleigh signal; an opticaldetector device comprising at least one single-photon detector (SPD) fordetecting the backscattered Rayleigh signal received over the fiberoptic cable; circuitry to produce an acoustic signal representative of adownhole property based on a phase of the backscattered Rayleigh signal,wherein the phase includes an in-phase (I) component and quadrature (Q)component; and a routing circuit to route the backscattered Rayleighsignal to provide at least a second backscattered Rayleigh signal when acount of the number of photons in the backscattered Rayleigh signalreaches or exceeds a threshold number of photons.
 17. An opticaldetection system comprising: a fiber optic cable to be disposed in adownhole location, the fiber optic cable including a sensing location togenerate a backscattered Rayleigh signal representative of measurementparameters; a light source, at a surface of the Earth, to transmit ameasurement signal to cause the sensing location to provide thebackscattered Rayleigh signal; an optical detector device comprising atleast one single-photon detector (SPD) for detecting the backscatteredRayleigh signal received over the fiber optic cable; circuitry toproduce an acoustic signal representative of a downhole property basedon a phase of the backscattered Rayleigh signal, wherein the phaseincludes an in-phase (I) component and quadrature (Q) component; and amonitoring unit coupled to an output of a superconducting nanowire SPD(SNSPD), the monitoring unit having an impedance lower than an impedanceof a non-superconducting region of the SNSPD, the monitoring unitconfigured to provide measurement data of the SNSPD.
 18. An opticaldetection system comprising: a fiber optic cable to be disposed in adownhole location, the fiber optic cable including a sensing location togenerate a backscattered Rayleigh signal representative of measurementparameters; a light source, at a surface of the Earth, to transmit ameasurement signal to cause the sensing location to provide thebackscattered Rayleigh signal; an optical detector device comprising atleast one single-photon detector (SPD) for detecting the backscatteredRayleigh signal received over the fiber optic cable; and circuitry toproduce an acoustic signal representative of a downhole property basedon a phase of the backscattered Rayleigh signal, wherein the phaseincludes an in-phase (I) component and quadrature (Q) component, whereinthe light source is configured to transmit a measurement signal havingan energy level below a threshold energy level such that eachbackscattered Rayleigh signal has an energy below a saturation energylevel of each of the plurality of SPDs.